The best way to get to know some Cologners is to live in a shared flat, also called "WG" in German. Sharing a flat has many advantages; you can cook and eat together, or party, and it's a good way to live cheaply for a short time or for an extended stay. Some shared flats are smaller with 2 roommates, whereas some have up to 10 roommates.
You can find shared flats as well as private rooms on the following websites:
www.easywg.de Extensive portal for finding flats or shared flats, rideshare opportunities (by car and bus), as well as student exchanges.